1. Field of the Invention
This invention relates generally to softening oilfield-produced water for steam flood operations and more particularly to treating waste brine generated by the regeneration step in a water softener system to obtain brine that can be utilized as the regeneration feed for the water softener step.
2. Background of Prior Art
Steam is often injected into subsurface formations, usually through injection wells, to recover heavy oil (oil with high viscosity), to reduce the viscosity so that oil will flow from injection well to the production well. Oilfield-produced water is the common source of available water in the oilfields and is thus utilized to generate steam at the surface, which is injected into one or more injection wells drilled into the subsurface formation. The steam reduces the viscosity of the heavy oil, causing it to flow to the one or more production wells in the oilfield.
Oilfield-produced water contains high levels of total dissolved solids (xe2x80x9cTDSxe2x80x9d). For example the TDS level of water from the Duri Field is about 4000 ppm and that from the Bakersfield, Calif. field is about 10,000 ppm, which is primarily due to the presence of high concentrations of calcium carbonate (CaCO3) and sodium chloride (NaCl).
However, commercially used steam generators require feed water with one (1) ppm hardness as CaCO3. To achieve such low hardness, two-stage water softeners are usually utilized for treating the oilfield-produced water. Such water softeners contain a primary softener followed by a secondary softener. Each softener contains heavy concentrations of sodium chloride (xe2x80x9csaltxe2x80x9d) in a suitable resin. These softeners are periodically regenerated. To obtain the one (1) ppm hardness leakage, counter-current regeneration with high salt loading, about 15 lbs. of salt per cubic foot of resin is typically adopted. With such high salt loading, the regeneration efficiency is very lowxe2x80x94in the range of twenty to twenty-five percent (20-25%). The regeneration solution leaving the softener (xe2x80x9ccomposite wastexe2x80x9d or xe2x80x9cwaste brinexe2x80x9d) is discarded. However, due to the low regeneration efficiency, the regenerated waste brine contains large amounts of salt, which in the prior art methods is wasted. Accordingly these softeners require use of very large amounts of salt, the majority of which is wasted. The regenerated waste brine with its high salt content also is not environmentally friendly.
The present invention provides a method of treating the waste brine generated by the regeneration step in a water softener system, preferably at least two stages, and for recovering brine that can be utilized as regeneration feed brine, thereby requiring substantially lesser amounts of fresh salt. In the present invention, there is no need to lower the pH of the brine before passing it through one or more nano-filters to reject polyvalent ions. As a result, the pH of the recovered brine does not need adjustment before it can be used.
The present invention provides a method of recovering usable brine from waste brine produced during regeneration of a softener by passing brine comprising a first concentration of monovalent ions through a softener to replace bivalent ions in said softener with monovalent ions, said softener discharging waste brine comprising bivalent ions and a second concentration of said monovalent ions and passing said waste brine from said softener through a nano-filter to remove most of the bivalent ions from said waste brine while passing most of the monovalent ions through said nano-filter, thereby obtaining recovered brine with said second concentration of monovalent ions. There is no need to lower the pH prior to passing through the nano-filter(s). Accordingly, there is no need to readjust the pH of the recovered brine to render it useful or usable.
The present invention also provides a method of counter-current regeneration of a water softener containing bivalent ions, by passing brine containing a first concentration of monovalent ions through a water softener to replace bivalent ions with a portion of the monovalent ions, said softener discharging waste brine comprising bivalent and monovalent ions; passing the waste brine from said water softener through a nano-filter to remove most of the bivalent ions therefrom, thereby obtaining fluid comprising monovalent ions (xe2x80x9ctreated brinexe2x80x9d); and by passing fluid obtained from said nano-filter through the water softener to further replace said bivalent ions in said water softener with monovalent ions from said treated brine to regenerate said water softener. No adjustment of pH of the brine prior to or after passing through the nano-filter(s) is needed for the present invention.
The method may further comprise treating said treated brine to obtain usable brine having ten to fifteen percent (10-15%) salt concentration. Another aspect of the present method may further comprise combining said treated brine with fresh brine prior to passing said treated brine through the water softener.
One preferred embodiment of the method may include passing the waste fluid through said nano-filter to produce a treated brine having from about one thousand (1000) to about five hundred (500) ppm of bivalent ions (such as Ca), also referred to as xe2x80x98hardnessxe2x80x99. Another feature of the inventive method may include of passing the fluid through the nano-filter may further include passing said fluid through at least two-stages of nano-filters, each said stage adapted to recover a portion of monovalent ions contained in said fluid.
Another aspect of the present invention regards a system of recovering salt from waste generated during counter-current regeneration of a water softener. This aspect of the present invention may use a source of fresh brine containing a selected concentration of salt, said fresh brine being fed to a water softener to replace bivalent ions in said softener with salt, said softener discharging waste brine containing bivalent ions and a portion of said salt; a nano-filter receiving said waste brine under pressure, said nano-filter removing the bivalent ions and passing at least a portion of said salt therethrough to provide treated brine suitable for use in the regeneration of said water softener. A further feature of this system may use a nano-filter with at least two-stages, each said stage recovering at least a portion said salt.
Examples of the more important features of the invention have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.